This application claims priority from German Patent Application 101 15 152.7 filed Mar. 27, 2001, and incorporates the contents of that application herein by reference.
The present invention relates to a method for distance measurement by means of transit time measurement of laser pulses.
A measuring instrument (distance sensor) for carrying out this method is described in German Patent document DE 30 20 996 A1, for example. A laser pulse is generated by a semiconductor laser as a transmitter with an upstream pulse generator. The laser pulse reflected by a target, whose distance from the vehicle is to be measured, is detected with respect to time by a photodiode, preferably an avalanche photodiode with a downstream transit time measurement circuit. Between the turn-on (triggering) of the pulse generator at time T0 and the transmission of the laser pulse at time TS as the actual start signal, there is, however, a time delay xcex94T=TSxe2x88x92T0. This delay depends primarily on the components used, in particular the pulse generator used, with a choice of the semiconductor laser also being a factor. If this delay were constant, it would not be critical, since it could be determined once and then taken into account in the transit time measurement as a correction value. This is not the case, however, because the delay can vary sharply, especially as a result of temperature variations. Thus, when certain components are used, the delay over the temperature range relevant for vehicles can be between 100 and 200 nanoseconds, which results in a variation of 100 nanoseconds. As a result, the start time and therewith the transit time measurement would have an imprecision of 100 nanoseconds, which would ultimately mean an imprecision in the distance measurement of 15 meters.
For this reason, DE 30 20 996 A1 proposes coupling out a portion of the laser pulse through an optical waveguide in the direction of the receiving diode as a start reference signal. The provision of an optical waveguide for this purpose is, however, very involved and expensive.
German Patent document DE 197 31 754 A1 describes integration of the aforementioned distance sensor in a headlight of a vehicle. The problem of determining the precise start time for the transit time measurement is, however, not addressed.
It is an object of this invention to provide a method for distance measurement for vehicles by measuring transit time of laser pulses, which realizes a simple, cost-effective and reliable correction of a delay, between a triggering of a pulse generator and a time of light emission of the laser pulse.
According to principles of this invention, advantage is taken of the circumstance that, when a distance sensor of the aforementioned type is integrated into a headlight, a laser pulse must pass through a light-transmissive shield cover (also called a headlight lens), which is transparent to laser radiation. For the invention, the light-transmissive shield cover is not merely a transparent design element as it is in DE 197 31 754 A1. Rather, the invention exploits the knowledge that the light-transmissive shield cover is almost completely transparent to the laser radiation, in other words, a small portion of the laser pulse is reflected back into the headlight by the light-transmissive shield cover. With appropriate beam characteristics (slightly divergent laser beam) and adequate spatial proximity between a transmitting laser and a receiving diode, this portion of the laser pulse reflected by the light-transmissive shield cover also reaches the receiving diode as measurements have demonstrated. Because the portion of the laser pulse reflected by the light-transmissive shield cover is detected with respect to time, the time of transmission of the laser pulse (the point in time when the laser starts to emit) can be determined exactly, since the distance traveled by the reflected laser beam from the transmitting laser to the light-transmissive shield cover and back to the receiving diode is always the same as a result of the fixed geometric arrangement.
In the event that the reflection from the light-transmissive shield cover is not sufficient when a conventional light-transmissive shield cover is used, provision is made in one embodiment to use a light-transmissive shield cover that is structured to be at least locally reflective so as to achieve increased reflection of a portion of the laser pulse.
Alternatively or in addition to this measure, provision is made in an embodiment to increase the sensitivity of the receiver within a specific time window in order to detect a low intensity signal reflected by the light-transmissive shield cover.
The idea upon which the invention is based is not limited to integration of the laser distance sensor into a headlight. Rather, the basic idea is to equip an installation space in which the distance sensor is arranged, with a cover wall that is almost transparent to the laser radiation (only), whereby a transit time of a portion of the laser pulse reflected by the cover wall is then measured by the receiver, as a reference time signal for the distance measurement. In general, when the laser wavelength lies outside the visible spectrum, the cover wall does not have to be transparent to visible light. The cover wall serves to protect the distance sensor from dirty water, dust, stones, etc. Thus it is possible to integrate the distance sensor into the bumper of a vehicle, for example.